Single nucleotide polymorphisms (SNPs) are the most common type of genetic diversity in the human genome, occurring at a frequency of about one SNP in 1,000 nucleotides or less in human genomic DNA (Kwok, Ann. Rev. Genom. Hum. Genet., 2:235-258 (2001)). SNPs have been implicated in genetic disorders, susceptibility to different diseases, predisposition to adverse reactions to drugs, and for use in forensic investigations. Thus, SNP (or rare mutation) detection provides great potentials in diagnosing early phase diseases, such as detecting circulating tumor cells in blood, for prenatal diagnostics, as well as for detection of disease-associated mutations in a mixed cell population.
Numerous approaches for SNP genotyping have been developed based on methods involving hybridization, ligation, or DNA polymerases (Chen et al., Pharmacogenomics J., 3:77-96 (2003)). For example, allele-specific polymerase chain reaction (AS-PCR) is a widely used strategy for detecting DNA sequence variation (Wu et al., Proc. Natl. Acad. Sci. USA, 86:2757-2760 (1989)). AS-PCR, as its name implies, is a PCR-based method whereby one or both primers are designed to anneal at sites of sequence variations which allows for the ability to differentiate among different alleles of the same gene. AS-PCR exploits the fidelity of DNA polymerases, which extend primers with a mismatched 3′ base at much lower efficiency, from 100 to 100,000 fold less efficient, than that with a matched 3′ base (Chen et al., Pharmacogenomics J., 3:77-96 (2003)). The difficulty in extending mismatched primers results in diminished PCR amplification that can be readily detected.
The specificity and selectivity of AS-PCR, however, is largely dependent on the nature of exponential amplification of PCR which makes the decay of allele discriminating power rapid. Even though primers are designed to match a specific variant to selectively amplify only that variant, in actuality significant mismatched amplification often occurs. Moreover, the ability of AS-PCR to differentiate between allelic variants can be influenced by the type of mutation or the sequence surrounding the mutation or SNP (Ayyadevara et al., Anal. Biochem., 284:11-18 (2000)), the amount of allelic variants present in the sample, as well as the ratio between alternative alleles. Collectively, these factors are often responsible for the frequent appearance of false-positive results, leading many researchers to attempt to increase the reliability of AS-PCR (Orou et al., Hum. Mut., 6:163-169 (1995); Imyanitov et al., Biotechniques, 33:484-490 (2002); McKinzie et al., Mut. Res., 517:209-220 (2002); Latorra et a, Hum. Mut., 22:79-85 (2003)).
Another technology involving probe hybridization methods used for discriminating allelic variations is TaqMan® genotyping. However, like AS-PCR, selectivity using this method is limited and not suitable for detecting rare (1 in ≧1,000) alleles or mutations in a mixed sample.
As such, there is a need in the art for improved compositions and methods to detect single point substitutions (e.g., SNPs), insertions, or deletions against a background of wild-type allele in thousand-fold or greater excess with increased sensitivity and specificity. The present invention satisfies this need and provides related advantages as well.